1. Field of the Invention
This invention relates to a liquid crystal medium useful as a material for an optical device, and in particular, to a liquid crystal medium having a wide temperature range of a liquid crystal phase, a large dielectric anisotropy and a large refractive index anisotropy. This invention also relates to an optical device using the liquid crystal medium, and particularly to an optical device that can be used in a wide temperature range and driven at a low voltage and can achieve a high-speed electrooptical response.
2. Description of Related Art
Liquid crystal display (LCD) devices utilizing liquid crystal compositions are widely used in displays of clocks, calculators, word processors and so on. These LCD devices utilize the optical anisotropy and dielectric anisotropy, etc. of liquid crystal compounds. The operation modes of the LCD devices mainly include phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), and vertical alignment (VA) and so on, which utilize one or more polarizers for display. Moreover, in recent years, more attention has been paid to the mode where an electric field is applied to an optically isotropic liquid crystal phase to induce electric birefringence (Patent References 1-9/Non-patent References 1-3).
Moreover, wavelength tunable filters, wavefront control devices, liquid crystal lenses, aberrational correction devices, aperture control devices and optical head devices etc. that utilize the electric birefringence in a blue phase as one of the optically isotropic liquid crystal phases have been proposed (Patent References 10-12).
According to the driving mode, LCD devices can be classified into passive matrix (PM) and active matrix (AM) types. The PM type is further classified into static type and multiplex type etc., and the AM type is further classified into thin film transistor (TFT) type, metal insulator metal (MIM) type and so on.
Such LCD devices contain liquid crystal compositions having suitable properties. To improve the characteristics of an LCD device, the liquid crystal composition preferably has suitable properties. A liquid crystal compound as a component of a liquid crystal composition needs to have the following general properties:    (1) chemical stability and physical stability;    (2) a high clearing point (phase transition temperature from the liquid crystal phase to the isotropic phase);    (3) a low lower-limit temperature of a liquid crystal phase (nematic phase, cholesteric phase, smectic phase, optically isotropic liquid crystal phase like blue phase, etc.);    (4) good compatibility with other liquid crystal compounds;    (5) a suitable dielectric anisotropy; and    (6) a suitable optical anisotropy.
Particularly, for an optically isotropic liquid crystal phase, a liquid crystal compound having a large dielectric anisotropy and a large optical anisotropy is preferred from the viewpoint of lowering the driving voltage.
When a composition containing a liquid crystal compound having chemical and physical stability (1st property) is used in an LCD device, the voltage holding ratio is improved.
Further, a liquid crystal composition containing a liquid crystal compound with a high clearing point or a low lower-limit temperature of a liquid crystal phase (2nd and 3rd properties) can have a wide temperature range of a nematic phase or optically isotropic liquid crystal phase, and thus can be used in a display device in a wide temperature range. A liquid crystal compound is generally mixed with a number of other liquid crystal compounds to prepare a liquid crystal composition, so as to exhibit better properties that are difficult to develop by a single compound. Therefore, a liquid crystal compound having good compatibility with other liquid crystal compounds (4th property) is preferably used in an LCD device. In recent years, LCD devices with superior properties, especially display performances like contrast, display capacity, response time and so on, are required. In addition, regarding the liquid crystal material used, a liquid crystal composition having a low driving voltage is required. Further, in order to drive at low voltage an optical device that is driven in an optically isotropic liquid crystal phase, a liquid crystal compound with a large dielectric anisotropy and a large optical anisotropy is preferred.
The optically isotropic polymer/liquid crystal composite materials disclosed in Patent References 1-3 and Non-Patent References 1-3 require high voltages for device operation. Patent References 4-9 disclose optically isotropic liquid crystal compositions and polymer/liquid crystal composite materials being expected to have lower operation voltages; however, the optically isotropic liquid crystal composition and polymer/liquid crystal composite material containing a compound having chlorophenyl moiety according to this invention was not mentioned therein.
[Patent Reference 1] Japanese Patent Publication No. 2003-32796
[Patent Reference 2] International Publication No. 2005/90520
[Patent Reference 3] Japanese Patent Publication No. 2005-336477
[Patent Reference 4] Japanese Patent Publication No. 2006-89622
[Patent Reference 5] Japanese Patent Publication No. 2006-299084
[Patent Reference 6] Japanese Patent Publication No. 2006-506477
[Patent Reference 7] Japanese Patent Publication No. 2006-506515
[Patent Reference 8] International Publication No. 2006/063662
[Patent Reference 9] Japanese Patent Publication No. 2006-225655
[Patent Reference 10] Japanese Patent Publication No. 2005-157109
[Patent Reference 11] International Publication No. 2005/80529
[Patent Reference 12] Japanese Patent Publication No. 2006-127707
[Non-Patent Reference 1] Nature Materials, 1, 64, (2002)
[Non-Patent Reference 2] Adv. Mater., 17, 96, (2005)
[Non-Patent Reference 3] Journal of the SID, 14, 551, (2006)